Exercises: Basics of Network Layer Experiential Learning Workshop

Transcription

1 Exercises: Basics of Network Layer Experiential Learning Workshop 1 General Guidelines 1. Make a team of two or three unless stated otherwise. 2. For each exercise, use wireshark capture to verify contents 3. Ensure to use proper capture filter and don t capture irrelevant traffic 4. Where appropriate or applicable, use wget or nc to access the web server. 5. The default client for accessing web server is assumed to be browser, preferably firefox. You can use Chrome or any other browse as well. 6. The webserver in the example below is taken as myweb.com. Please use your hostname or corresponding IP address instead in your exercise. 7. To kill any program in the linux terminal, please press Ctrl-C and not Ctrl-Z. The latter will suspend the program and not stop it. Note: Appendix provides instructions on installing any package if not already installed. 2 IP Routing 2.1 Basic Routing your team number e.g. 1 for team 1, 2 for team 2 and so on. To get a second interface on B, use the USB to ethernet adaptor on B. 2. Define IP addresses for B. Assign the IP addresses via the network Setting UI or via the following command. a. sudo ip addr add 10.x.1.101/24 dev eno1 b. sudo ip addr add 10.x.3.1/24 dev enx (USB device) 3. Convert machine B into a router a. sudo sysctl -w net.ipv4.ip_forward=1 4. Define routing for network /24 on Ha. a. sudo ip route add 10.x.3.0/24 via 10.x Define routing for network /24 on Hc a. sudo ip route add 10.x.1.0/24 via 10.x Check reachability between Ha and Hc (use ping to check) RPR/KSIT Basics of Network Layer Page 1 of 5

2 2.2 TTL Expiry 1. Connect 3 m/cs in a network as in above exercise of PMTU discovery. 2. Send a ping packet A to C with TTL=1. 3. Analyze the response with ICMP errors. Analyze how TTL Expiry works. 4. Analyze the source IP address in the ICMP error packet. 2.3 IP fragmentation 1. Connect 3 m/cs in a network as in above exercise of PMTU discovery. 2. Change the MTU size of the link to a smaller value. 3. Send a large size UDP packet i.e packet size larger than MTU a../udp_client -b s 10.x p c 1 4. Analyze the packet transmission in wireshark and see the fragmentation. 3 Hands-on 3: ICMP Errors Note: The IP addresses specified in the below example are for illustrative purposes only. Use the IP Address assigned to your machines in the network. For this exercise you need to make a group of ICMP Redirect your team number e.g. 1 for team 1, 2 for team 2 and so on. The exercise can work without the use of small 4/8 port switch separately i.e. this exercise can be carried out in regular lab network. 2. On B and C, assign addresses from a different network on the loopback (lo) interface? a. sudo ip addr add 10.x.2.101/24 dev lo on B b. sudo ip addr add 10.x.3.201/24 dev lo on C 3. Disable acceptance of ICMP redirect on A to ensure that its routing table is not updated as per ICMP redirect. It is assumed that ethernet interface on the machine is eth0 in the examples below. Replace it with appropriate values. a. sudo sysctl -w net.ipv4.conf.all.accept_redirects=0 b. sudo sysctl -w net.ipv4.conf.default.accept_redirects=0 RPR/KSIT Basics of Network Layer Page 2 of 5

3 c. sudo sysctl -w net.ipv4.conf.eth0.accept_redirects=0 4. Add mis-redirected (incorrect) routing entries on A. a. sudo ip route add 10.x.2.0/24 via 10.x b. sudo ip route add 10.x.3.0/24 via 10.x Ping from A to addresses 10.x and 10.x Analyze in wireshark the icmp redirect packets. Use the capture filter icmp. 3.2 PMTU Discovery your team number e.g. 1 for team 1, 2 for team 2 and so on. To get a second interface on B, use the USB to ethernet adaptor on B. 2. Convert machine B into a router as well as change the MTU value of the link between B and C. a. sudo ip link set dev eth1 mtu 1000 b. sudo sysctl -w net.ipv4.ip_forward=1 3. Define appropriate routing on A to reach C a. sudo ip route add 10.x.3.0/24 via 10.x Define appropriate routing on C to reach back A a. sudo ip route add 10.x.1.0/24 via 10.x Send a ping packet bigger than 1000 bytes from A to C. Run following on A a. ping -c1 -s p x Analyze wireshark capture to study ICMP error (fragmentation needed) and subsequent packet fragmentation (This will be studied later). How many packets are finally generated from A. Analyze the ping response. Are there two responses? 3.3 Exercise Expectations 1. Able to understand ICMP errors and source address of packet corresponding to ICMP errors. 2. Able to understand ICMP Redirect, PMTU Discovery and TTL expired. RPR/KSIT Basics of Network Layer Page 3 of 5

4 4 Hands-on 4: Understanding ARP 4.1 Study ARP table maintenance 1. Setup three machines A, B, and C as shown in ( 3.1 ICMP Redirect). In this exercise we do not need 2 nd and 3 rd network i.e,. 10.x.2.0/24 and 10.x.3.0/24 and thus these addresses need not be assigned. Use of separate small switch is not recommended for this exercise 2. Study the existing ARP table and identify which machines are these. a. arp -an 3. Ping one live machine (i.e. machine which is reachable) and one unreachable machine. Study the ARP table. What does the entry show for unreachable machine. 4. Ping the broadcast address of your network. This requires sudo privilege. a. sudo ping -b -c2 10.x Analyze in wireshark on number of ICMP response received as well as number of entries added in ARP Table. 4.2 Study Gratuitous ARP 1. Use the same setup as above. Note down MAC Address of 3 machines (A, B & C). 2. Install arping on the machine A (and may be others) a. sudo apt install arping 3. On A, add the IP address of A to that of C (replace eth0 with applicable interface name) a. sudo ip addr add 10.x.1.201/24 dev eth0 4. Issue arping with this new address to B a. sudo arping -s 10.x c 2 10.x Study the ARP table at B. It should show updated MAC address of A for IP Address of C. 4.3 Exercise Expectations 1. Able to understand the use of ARP and Gratuituous ARP. 5 ICMP Errors Note: The IP addresses specified in the below example are for illustrative purposes only. Use the IP Address assigned to your machines in the network. For this exercise you need to make a group of 3. RPR/KSIT Basics of Network Layer Page 4 of 5

5 5.1 Proxy ARP 1. Connect 3 m/cs in a network as shown in diagram below. Assign the address manually to these systems Use the mask of /22 to ensure that both Ha and Hc are in same network. 2. Define the subnet mask for router interface to be /24 to ensure that Ha is network-1 and Hb is in network Enable proxy ARP on Hb (router) a. sudo sysctl -w net.ipv4.conf.all.proxy_arp=1 b. sudo sysctl -w net.ipv4.ip_forward=1 4. No need to define routing entries at A and C since they are part of same network. Clear up their ARP table (sudo arp -d <IP address) for each IP Address. 5. Check reachability and check ARP table. Analyze the ARP Response in wireshark. ARP Reply should come from Hb for both Ha and Hc though both Ha and Hc do not know about this router. 5.2 Exercise Expectations 1. Able to understand proxy ARP, Routing, and ICMP errors and source address of packet corresponding to ICMP errors. ß end of exercise handout à RPR/KSIT Basics of Network Layer Page 5 of 5